A quartz pendulous accelerometer, or a quartz accelerometer for short, is a force balance type acceleration sensor. It has been widely used in inertial navigation, measurement while drilling (MWD) and measurement while logging (LWD) in oil and gas exploration. Compared with accelerometers employing other working principles, the quartz pendulous accelerometer accounts for most of the market share in the fields of inertial navigation, MWD and other fields due to its advantages in terms of price, precision, environmental adaptability and other performance characteristics.
The quartz pendulous accelerometer is mainly composed of an acceleration sensing device, i.e., the quartz meter, a servo processing circuit, and an output circuit. The quartz meter includes a quartz pendulum, a torquer yoke, a torquer coil, and a magnet. Among them, the quartz pendulum and end faces of the torquer yoke make up a differential capacitors with the upper gold-plated face and the lower gold-plated face of the quartz pendulum acting as movable polar plates of the differential capacitors while the torquer yoke act as stationary polar plates of the differential capacitors. After receiving an external acceleration signal, the quartz pendulum swings due to an inertial force so that capacitance values of the differential capacitors change. The change in the differential capacitance is converted into a change of a voltage signal by means of a Capacitance to Voltage (C-V) readout circuit of a servo circuit. A signal conditioning circuit then outputs a driving current to a torquer through the torquer coil to create a balancing force (balancing torque), which counter-balances the inertial force (inertial torque) generated by the external inertia acceleration so as to reach the balance of force in the closed-loop system. Further, a magnitude of the feedback current input to the coil is directly proportional to the value of the input inertia acceleration so that the acceleration can be calculated.
The traditional servo processing circuit includes a C-V readout circuit, a proportional-integral-derivative (PID) control circuit, a transconductance amplifier, a feedback circuit and other modules, which mainly accomplishes the conversion of the physical properties (C-V), signal conditioning of the closed-loop system under static and dynamic conditions, voltage-current conversion, and the driving ability.
The servo processing circuit includes an analog negative feedback circuit, in which an analog current is employed in a feedback current driving mode and a pulse servo negative feedback circuit, in which a pulse current is employed in a feedback current driving mode. In addition, the pulse current in turn includes a width-modulated pulse, an intermittent pulse, etc.
The output circuit (digital quantization) is mainly to solve a digitization problem of the accelerometer. Currently, the traditional output circuit adopts a current-frequency (I-F) conversion circuit or an analog/digital (A/D) conversion circuit solution. The I-F conversion circuit solution accomplishes measurement digitization) of the output current using an integrator and a constant-current source to convert a current signal into a frequency signal, so as to facilitate navigation calculation by a navigation computer. On the other hand, the A/D conversion circuit converts the output current signal into a voltage signal by means of a sampling resistor, and further convert the voltage signal into a digital signal using an analog-to-digital conversion (ADC) chip.
One of the existing technologies uses the capacitance detection, an analog PID control strategy, and an analog negative feedback solution. Digital quantization is realized using an I-F conversion circuit or an A/D conversion circuit. There are the following problems:
1. Circuit scale and cost: before the navigation calculation is performed, the analog output needs to be converted into a digital quantity. The common solution at present is to perform the I-F conversion. Since this method does not require a sampling resistor in the closed-loop system, the I-F conversion circuit has no effect on the measurement range or internal parameters. Moreover, the I-F conversion circuit and the servo circuit are relatively independent and do not affect each other. However, I-F design parameters and the levels of precision directly determine the overall precision level of the quartz pendulous accelerometer. As the I-F conversion circuit is relatively complex in scale, its level of precision extremely susceptible to changes in the ambient temperature and its own parameters, which makes it difficult for system integration and miniaturization or cost reduction.
2. System response: When the output is analog signals, the feedback bandwidth is significantly restricted. In order to achieve fast responses, it is necessary to increase the bandwidth, which would increase the noise level and negatively impact the performance of the whole system, affecting the overall performance level;
3. Dynamic error: When using analog feedback loop, the decrease in the electrical stiffness of the system in the working frequency band reduces the precision of the dynamic response. This is because the electric stiffness and response frequency are correlated, where with the increase in the response frequency, the electric stiffness decreases, resulting in reduced dynamic response precision. In contrast, the present disclosure employs an oversampling digital feedback technique is employed, which increases the electric stiffness of the system within the effective frequency band so that the dynamic response precision of the system is improved.
4. Digital quantization precision: The feedback signal of a PDM (pulse density modulation feedback) accelerometer is a series of pulses with constant amplitudes. The force produced by the torquer, exerted on a movable mass block, is a series of pulses with a constant amplitudes. Each pulse represents an accurate input acceleration increment. When using analog feedback in the accelerometer, the analog output requires analog-to-digital conversion. For example, the I-F conversion circuit is commonly used in the inertial navigation, the ADC conversion is commonly used in the industrial field; I-V-D: a current is converted to a voltage, and then the voltage signal quantity is digitalized). While in a digital feedback accelerometer, the analog-to-digital conversion process is accomplished in an accelerometer system loop. The digital feedback is the control signal and the feedback signal of the system, and thus the error of the analog-to-digital conversion is small.
5. Linearity: the linearity of the traditional analog negative feedback is mainly limited by the linearity of the torquer. In a full-scale range, the driving current change range of the torquer is very large, for example, for a quartz pendulous accelerometer with a measurement range of ±30 g and a scale factor of 1.2 mA/g, in order to distinguish the external input acceleration value of 1 μg, the required torque feedback driving current is 1.2 mA*10−6; and when the external input acceleration is 30 g, the feedback current is 1.2 mA*30, the requirement for the linearity of the torquer in such a large dynamic change range. In addition, the current amplification capability of the constant-current source is tested. The precision of the constant-current source and the linearity of the torquer determine the linearity of the accelerometer. For the digital feedback control, the input acceleration value is modulated into a pulse torque with high speed and a constant amplitude and width, and the input acceleration is quantified into pulse density of the output. As such, the nonlinear problem of the large dynamic current of the torquer is avoided.
In another of the existing technology, pulse density modulation (PDM) or pulse width modulation (PWM) negative feedback is adopted, and the following problems:
This technology is still based on the Nyquist sampling law, and its overall control strategy is still based on the traditional analog feedback solution, and therefore, some dynamic characteristic defects of the analog negative feedback such as dynamic precision and system response still exist. The problem of quantized noise has not been solved, the noise shaping is not achieved, the quantized noise of the digital output is relatively large, the number of bits of the digitized output is not enough, or the precision of the system is lost after the digital quantization.
There is a Dead-Zone or Idle Tones problem that is, the pulse density feedback of this technology, due to low electric stiffness of the system, when the accelerometer is in a weak input signal mode, the output is prone to the instability caused by the ring oscillator noise.